Ain't silly question!?

[DrAmir0078]

I am confused, how can you precisely know the dialer % of the any vaporizer to give you the exact MAC?
We have here like some advanced - other than Drager too - anesthesia machine which I only found out MAC delivered to the patient!
Is there any equation?
A way?
To relieve the confusion!

Routinely (with other machines), we set Isoflurane on 1.5 %, Sevoflurane on 2 %, Halothane (still somewhere used) at 1%

In the picture shows the arrows
Sevoflurane 1% delivered 0.6 MAC in Low Flow Anesthesia (0.5 L of Oxygen) - this is an advanced German made Anesthesia Machine.


Thanks

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[norski]

Won't the apparatus just use the measurements from insp(gas) vs exp(gas),based on the metrics of your patient,then calculate the MAC? If the yellow vaporizer is sevo,your 1% on the dial seems to be just about consistent with the 0.6 MAC on the monitor.

 

[btbam]

You need to set your vaporizer dial higher if you’re doing low flows. I assume you meant 0.5 L of oxygen?

 

[DrAmir0078]

I corrected it, thanks a lot.
But still didn't get it about exact MAC vs dialer

 

[DrAmir0078]

Even though, I didn't get it honestly.
Although the majority of our Machines are old enough not to have such sophisticated insp vs exp gas!
So how to do the calculation?

How about if it is not LFA, we set the fresh O2 to 7 Lt for an adult (because we don't have in some machines an absorbent and even scavenger system) - discussed before!

I just need to know how to set the dial % for the exact insp MAC needed!?

Won't the apparatus just use the measurements from insp(gas) vs exp(gas),based on the metrics of your patient,then calculate the MAC? If the yellow vaporizer is sevo,your 1% on the dial seems to be just about consistent with the 0.6 MAC on the monitor.

 

[btbam]

I’m not sure exactly what you’re asking. If your machine and vaporizer are calibrated correctly you *should* deliver whatever it’s set at. Obviously the Et agent will always be slightly less the Fi. If the inspired agent is way off from the dial then you need someone to service the machine and/or vaporizer.

 

[DrAmir0078]

OK,
If I set it up the Isoflurane on 1.5 % how much exactly the MAC delivered ?

I’m not sure exactly what you’re asking. If your machine and vaporizer are calibrated correctly you *should* deliver whatever it’s set at. Obviously the Et agent will always be slightly less the Fi. If the inspired agent is way off from the dial then you need someone to service the machine and/or vaporizer.

 

[norski]

This machine of yours,at least the one in your picture DOES measure et and in sevo!

0.5l/min and 1% sevo with 0.6 MAC looks like you're either using a test lung or at the very end of a case and you've just dialled down the sevo. Can't read the numbers,up on the left of your screen.

I understand your need to know, but I can't off the top of my head give you the calculus for figuring out MAC!

At MRi we use an old Dräger Fabius machine with a sevo vaporizer only,no MAC or in/et calculations. We just head for 2% to get approximately 1 MAC.

 

[DrAmir0078]

Yes, it was at the end of the operation - true guess - we were switching from Iso to Sevo (it was pediatric case)

So, you are like us, set the Sevo on 2% for roughly estimation of 1 MAC !

This machine of yours,at least the one in your picture DOES measure et and in sevo!

0.5l/min and 1% sevo with 0.6 MAC looks like you're either using a test lung or at the very end of a case and you've just dialled down the sevo. Can't read the numbers,up on the left of your screen.

I understand your need to know, but I can't off the top of my head give you the calculus for figuring out MAC!

At MRi we use an old Dräger Fabius machine with a sevo vaporizer only,no MAC or in/et calculations. We just head for 2% to get approximately 1 MAC.

 

[norski]

Yes, I see now that your machine detects a secondary gas,the MAC makes a lot more sense now.

I usually refer to either Textbook of Anaesthesia or Fundamentals of Anaesthesia for MAC values with inhalational agents. Think at least one of them is part of the FRCA curriculum (I might be mistaken, but you're the equivalent or same as a British/Commonwealth annaesthesia registrar in Iraq,right?).

Yes, it was at the end of the operation - true guess - we were switching from Iso to Sevo (it was pediatric case)

So, you are like us, set the Sevo on 2% for roughly estimation of 1 MAC !

 

[MirrorTodd]

Theoretically just divide the percent concentration delivered by the percent concentration for 1 MAC of whatever gas you are using to get the MAC percentage for your set concentration. Example: iso at 0.8% with 1.15% iso being 1 MAC would be 0.8%/1.15%=0.7 MAC
EDIT: not accounting for age/other variables that change MAC of course.

 

[DrAmir0078]

Yes, our program The Iraqi Residency - Board - in Anaesthesia is eligible for admission to FRCA exams (it is a recognized accredited program)

Still we love to hang out with Barash, Morgan and the scary Miller too!

Yes, I see now that your machine detects a secondary gas,the MAC makes a lot more sense now.

I usually refer to either Textbook of Anaesthesia or Fundamentals of Anaesthesia for MAC values with inhalational agents. Think at least one of them is part of the FRCA curriculum (I might be mistaken, but you're the equivalent or same as a British/Commonwealth annaesthesia registrar in Iraq,right?).

 

[DrAmir0078]

OK neat and many thanks!

So MAC is = Insp Gas % / Dial %

Another example

So Insp Gas = MAC X Dial

Iso inspired = 0.7% MAC X 1.15 % dial = 0.8 % inspired

So half of the equation

Iso inspired = 0.35 MAC X 0.56 % dial = 0.2 % Inspired


So - Finally Multiply Above by 3 (to get exact MAC)

Iso inspired = 1.05 MAC X 1.68 % dial = 1.76 % inspired

So I am setting from now on the Isoflurane at 1.68% to get 1 MAC

Great... Am I right?


In Sevoflurane

Sevoflurane = 1 MAC X 2 % dial = 2 % inspired (is it real?)

hmmm and in pediatrics The MAC is going to be different!

Theoretically just divide the percent concentration delivered by the percent concentration for 1 MAC of whatever gas you are using to get the MAC percentage for your set concentration. Example: iso at 0.8% with 1.15% iso being 1 MAC would be 0.8%/1.15%=0.7 MAC

 

[Mman]

I think you are asking about the difference between the setting on the vaporizer dial and the displayed MAC of the gas based off measured end tidal concentration of gas. Those are 2 related but different things and there is no magic formula to convert 1 to the other because it depends. How I think about it....

The dial on the vaporizer is merely controlling the concentration of anesthetic gas delivered into the total fresh gas flow you are delivering. So if you are at 0.5L/min of total fresh gas flow with the dial set on 2% sevoflurane, those 500 mls every minute have 2% sevoflurane in them. The thing is, your patient is probably having a minute ventilation > 0.5 L/min, probably something closer 5 L/min. The other 4.5 L/min of gas they are breathing is their exhaled gas that went through the CO2 absorbent. So if you just started the case and the patient has no sevoflurane in them, 90% (4.5L/min) of what they are breathing has 0% sevoflurane and 10% (0.5 L/min) has a concentration of 2% sevoflurane. That means the total 5 L/min of gas they are breathing has only roughly 0.2% sevoflurane compared to your vaporizer setting of 2%. When they exhale there is still almost no sevoflurane so their MAC will be calculated at 0.0. If you keep your flows up a little and reach a steady state where the amount of sevo being inhaled and exhaled is basically equal, then the calculated MAC will be much closer to matching what you see on the vaporizer dial.

The key thing to remember is that the lower your flows, the bigger the disconnect between the vaporizer dial setting and the actual MAC the patient is experiencing (at least until you end up at a steady state). It's also why at the end of the case you gotta crank up your flows to wake them up or else they will be just rebreathing the same sevo molecules over and over.

(and yes this is oversimplified and ignoring things like losses to scavenging and the gas sample line)

 

[Mman]

OK neat and many thanks!

So MAC is = Insp Gas % / Dial %

Another example

So Insp Gas = MAC X Dial

Iso inspired = 0.7% MAC X 1.15 % dial = 0.8 % inspired

So half of the equation

Iso inspired = 0.35 MAC X 0.56 % dial = 0.2 % Inspired


So - Finally Multiply Above by 3 (to get exact MAC)

Iso inspired = 1.05 MAC X 1.68 % dial = 1.76 % inspired

So I am setting from now on the Isoflurane at 1.68% to get 1 MAC

Great... Am I right?


In Sevoflurane

Sevoflurane = 1 MAC X 2 % dial = 2 % inspired (is it real?)

hmmm and in pediatrics The MAC is going to be different!

The MAC number on the machine is simply calculating the end tidal sevoflurane (or isoflurane) and comparing it to age adjusted norms.

age related MAC charts for Iso, Sevo, and Des

 

[MirrorTodd]

OK neat and many thanks!

So MAC is = Insp Gas % / Dial %

Another example

So Insp Gas = MAC X Dial

Iso inspired = 0.7% MAC X 1.15 % dial = 0.8 % inspired

So half of the equation

Iso inspired = 0.35 MAC X 0.56 % dial = 0.2 % Inspired


So - Finally Multiply Above by 3 (to get exact MAC)

Iso inspired = 1.05 MAC X 1.68 % dial = 1.76 % inspired

So I am setting from now on the Isoflurane at 1.68% to get 1 MAC

Great... Am I right?


In Sevoflurane

Sevoflurane = 1 MAC X 2 % dial = 2 % inspired (is it real?)

hmmm and in pediatrics The MAC is going to be different!

I don't know about inspiratory gas percentage. My equation (simplified) was dial %/known % for 1 MAC. Therefore. Iso 1.15% on dial divided by 1.15% known MAC of Iso equals 1 MAC of Iso, alternatively Sevo 2% on dial/2% known MAC of Sevo equals 1 MAC of Sevo

 

[DrAmir0078]

I am close to know it!
Many Thanks

I think you are asking about the difference between the setting on the vaporizer dial and the displayed MAC of the gas based off measured end tidal concentration of gas. Those are 2 related but different things and there is no magic formula to convert 1 to the other because it depends. How I think about it....

The dial on the vaporizer is merely controlling the concentration of anesthetic gas delivered into the total fresh gas flow you are delivering. So if you are at 0.5L/min of total fresh gas flow with the dial set on 2% sevoflurane, those 500 mls every minute have 2% sevoflurane in them. The thing is, your patient is probably having a minute ventilation > 0.5 L/min, probably something closer 5 L/min. The other 4.5 L/min of gas they are breathing is their exhaled gas that went through the CO2 absorbent. So if you just started the case and the patient has no sevoflurane in them, 90% (4.5L/min) of what they are breathing has 0% sevoflurane and 10% (0.5 L/min) has a concentration of 2% sevoflurane. That means the total 5 L/min of gas they are breathing has only roughly 0.2% sevoflurane compared to your vaporizer setting of 2%. When they exhale there is still almost no sevoflurane so their MAC will be calculated at 0.0. If you keep your flows up a little and reach a steady state where the amount of sevo being inhaled and exhaled is basically equal, then the calculated MAC will be much closer to matching what you see on the vaporizer dial.

The key thing to remember is that the lower your flows, the bigger the disconnect between the vaporizer dial setting and the actual MAC the patient is experiencing (at least until you end up at a steady state). It's also why at the end of the case you gotta crank up your flows to wake them up or else they will be just rebreathing the same sevo molecules over and over.

(and yes this is oversimplified and ignoring things like losses to scavenging and the gas sample line)

 

[DrAmir0078]

Yes, the link is awesome
Like in any chapter in those textbooks, but I am here near to say that 1 MAC value is a percentage = so my rubbish equations above = nothing is inspired gas % which is only determined by the machines - advanced one !

Many Thanks

The MAC number on the machine is simply calculating the end tidal sevoflurane (or isoflurane) and comparing it to age adjusted norms.

age related MAC charts for Iso, Sevo, and Des

 

[DrAmir0078]

Yup... I tried to get what you meant, thought you have looked at the picture (calculating both inspired 0.9 = so assuming 0.8) - my bad, I apologize

I am pretty close to get it !

I don't know about inspiratory gas percentage. My equation (simplified) was dial %/known % for 1 MAC. Therefore. Iso 1.15% on dial divided by 1.15% known MAC of Iso equals 1 MAC of Iso, alternatively Sevo 2% on dial/2% known MAC of Sevo equals 1 MAC of Sevo

 

[Mman]

Yes, the link is awesome
Like in any chapter in those textbooks, but I am here near to say that 1 MAC value is a percentage = so my rubbish equations above = nothing is inspired gas % which is only determined by the machines - advanced one !

Many Thanks

yes. MAC is a number calculated from the actual gas the patient exhales. The dial on the vaporizer and the measured inspired concentrations are determining how much anesthetic gas you are putting into the patient, but it's what they exhale that gets measured/calculated into the MAC number displayed on the screen.

 

[DrAmir0078]

This is awesome!
MAC what is exhaled, not inspired!

So importantly to know what MAC % for each gas exhaled!

yes. MAC is a number calculated from the actual gas the patient exhales. The dial on the vaporizer and the measured inspired concentrations are determining how much anesthetic gas you are putting into the patient, but it's what they exhale that gets measured/calculated into the MAC number displayed on the screen.

 

[DrAmir0078]

Yeah
So when I set the dial to 2 % Iso

2 % / 1.15 = 1.7 % MAC (which means higher)

So keeping our Iso (which is our routine) to

1.5 / 1.15 = 1.2 MAC

I feel strong now, did I get it right?

In children 1 year

If I set it on

1.5 / 1.49 = 1.006 MAC as they need more MAC than adults, so we need to adjust the dial up

If on 2 / 1.49 = 1.34



In Older age like 80 years old

1.5 / 0.91 = 1.57
So I need to lower the dial

0.9 / 0.9 = 1 MAC


What a Grace?

Ain't silly question!

I don't know about inspiratory gas percentage. My equation (simplified) was dial %/known % for 1 MAC. Therefore. Iso 1.15% on dial divided by 1.15% known MAC of Iso equals 1 MAC of Iso, alternatively Sevo 2% on dial/2% known MAC of Sevo equals 1 MAC of Sevo

 

[pgg]

Mman's excellent post above explains it well.

You should also read and understand this presentation on gas flows and how it affects end tidal % gas and especially FiO2 when using low flows. It addresses all of these issues very well.

http://etherweb.bwh.harvard.edu/education/PHILIP/Tech_Block_04/1_LowFlowO2Agt.pdf

 

[DrAmir0078]

Excellent,
Thanks pgg

Mman's excellent post above explains it well.

You should also read and understand this presentation on gas flows and how it affects end tidal % gas and especially FiO2 when using low flows. It addresses all of these issues very well.

http://etherweb.bwh.harvard.edu/education/PHILIP/Tech_Block_04/1_LowFlowO2Agt.pdf